Chemical instability and high impedance at the interface of Li-metal electrodes limit the
electrochemical performance of high-energy-density batteries. In order to overcome this
challenge, it is paramount to understand the composition of the interface of lithium and
electrolytes. The SEI layer chemistry is known to be highly complex and heterogeneous at the
sub-nanometer scale, the exact function of its constituents remains poorly understood. SEI layer
composition and mechanical properties affect coulombic efficiency, therefore it is paramount to
understand the passivation layer on lithium metal on the nm-scale to fully understand the
complex chemistry of the passivation layer. There have been numerous studies to analyze SEI on
Li metal anodes: the passivation layer consists of irreversibly-plated Li, also known as “dead Li”,
inactive ~100 nm-thick SEI layer, and the active passivation layer between Li metal and inactive
SEI

The thickness and domain size of SEI components dictates the use of instrumental
techniques beyond the diffraction limit. Generally, such techniques involve a high vacuum in
order to achieve such a resolution. Transmission and scanning electron microscopies, TEM and
SEM, have been used as tools to observe the nucleation process during Li plating and subsequent
growth of lithium dendrites, the main culprit in safety issues of LIB. However, more ambient
conditions are necessary to analyze the pristine initial SEI layer. However, with the recent
development of scattering-type Scanning Near-field Optical Microscopy (sSNOM), it is possible
to image the chemical nature of the SEI layer in sub-diffraction limit in ambient conditions,
which we applied to Li metal battery system.

Due to the reactivity of Li metal, it is important to ensure the cleanliness of the prepared
Li metal foil to avoid any potential contamination, especially from volatile organic compounds
(VOC), present in the majority of liquid battery electrolytes. In our study, we used battery grade
GEN 2 electrolyte (EC/EMC 1.2 M LiPF ₆ ). To address this issue, we have developed a transfer
method ensuring the pristine condition of the Li surface before coming in contact with the
electrolyte. A thin film, formed upon contact with the electrolyte on the Li metal surface
passivates the surface and presents as an initial SEI layer. By using nano-FTIR spectroscopy we
revealed the chemical composition of the GEN2/Li surface.

We have also shown the evolution of morphology and chemical composition of the SEI
layer beyond the diffraction limit, reaching 20 nm resolution. We have conducted our
experiments at Advanced Light Source (ALS) Beamline 2.4 (Synchrotron Infrared
Nanospectroscopy (SINS) and Imaging), using a bright and broadband infrared light source and a
detector that allows probing vibrations at a lower cutoff frequency ~350 cm-1, which opens a
unique opportunity to probe inorganic SEI components by nano-FTIR spectroscopy. We revealed
the evolution of the initial SEI layer in Li-metal anodes as well as the passivated surface of
plated Li “Dead Li”. Obtained knowledge brings us closer to the understanding of the early
evolution of the Li metal battery interface.

The work is supported by US DOE under contract no. DE-AC02-05CH11231